Porphyrin-naphthodiimide interactions as a structural motif in foldamers and supramolecular assemblies

Article abstract of DOI:10.1039/b804267e

π-π Stacking interactions between electron deficient naphthalenediimides (NDI) and electron-rich porphyrins (POR) leading to charge transfer are shown to be prevalent in linked NDI-POR and POR-NDI-POR structures. For flexibly-linked systems, intramolecular interactions lead to S-shaped foldamers in solution, whereas intermolecular association is predominant in more rigid systems. The foldamer structures can be interrupted by competing aromatic solvents, by six-coordination of metallated porphyrin derivatives, by protonation of the free base porphyrin in non-metallated structures, and in facially sterically hindered porphyrins. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b804267e

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Porphyrin–naphthodiimide interactions as a structural motif in foldamers and
supramolecular assemblies†
Zulkifli Merican, Ken D. Johnstone and Maxwell J. Gunter*
Received 14th March 2008, Accepted 16th April 2008
First published as an Advance Article on the web 16th May 2008
DOI: 10.1039/b804267e
p–p Stacking interactions between electron deficient naphthalenediimides (NDI) and electron-rich
porphyrins (POR) leading to charge transfer are shown to be prevalent in linked NDI–POR and
POR–NDI–POR structures. For flexibly-linked systems, intramolecular interactions lead to S-shaped
foldamers in solution, whereas intermolecular association is predominant in more rigid systems. The
foldamer structures can be interrupted by competing aromatic solvents, by six-coordination of
metallated porphyrin derivatives, by protonation of the free base porphyrin in non-metallated
structures, and in facially sterically hindered porphyrins.
polymer systems such as proteins and DNA.19 Although many
foldamers are based on well-defined relatively rigid systems,20
Introduction
flexible examples are less common.21 The NDI–DAN (DiAlkoxy-
Naphthalene) motif has been the subject of a detailed study in
the solution-state folding of dimers, trimers, and higher oligomers
of linked naphthalenediimides and dialkoxynaphthalenes.22 For
example DAN–NDI–DAN trimers have been shown to adopt
either intercalative, pleated or elongated structures, depending on
their primary structure, solvent and temperature.
Previously, we have described the synthesis of porphyrin-based
catenanes14 and rotaxanes15 whose templated assembly relied on
a presumed NDI–POR interaction. Subsequently,16 we reported
the reversible, thermodynamically-controlled self-assembly of a
series of rotaxanes, pseudorotaxanes and catenanes utilising
both crown-ether-naphthalenediimide host–guest interactions,
and metalloporphyrin–pyridyl coordination. For structures that
comprise a porphyrin linked covalently by polyoxyethylene chains
to a naphthalene diimide, which in turn is linked to a substituted
pyridine 1a, we specifically noted that there was clear evidence
of intramolecular interactions between the porphyrin and the
naphthalene diimide moieties as depicted in structure 1a.
Naphthalene diimides and porphyrins have been used extensively
both as structural and functional components in supramolecular
chemistry. Inparticular, the electron acceptor qualities andthe aro-
matic nature of N,N’-disubstituted 1,4,5,8-naphthalenediimides
(NDI)1 make them ideal candidates for use as building blocks
where face-to-face charge transfer stabilisation with a suitable
electron-rich aromatic donor component results in structurally
defined assemblies, which can in turn be used in the templated
construction of supramolecular systems. These include rotaxanes
and catenanes,2–4 including neutral, bi-stable redox- and cation-
driven rotaxanes5 and cation-templated pseudorotaxanes.6,7 Self-
stacking of the planar aromatic NDI molecules has also been the
major factor in self-assembled helical nanotubes.8 The reversible
nature of the intermolecular forces involved in the stacking motif
has also been used to advantage in a combinatorial approach to
supramolecular assembly.9
A study of intermolecular stacking of porphyrins has a long his-
tory, and porphyrin self-aggregation has become a well recognised
phenomenon in both aqueous and organic solvents.10
The combination of porphyrins with naphthodiimides and other
related diimides (including smaller pyromellitimides and larger
perylenediimides) has produced a wide variety of supramolecules
in both covalently11 and non-covalently12 linked systems. The
donor–acceptor nature of the resulting arrays has been utilised
to considerable effect in the production of dynamic13–16 and
controllable17 systems, both in solution and immobilised on
polymer supports.18
The p-stacking characteristics of NDI have been utilised to
advantage in the study of non-natural foldamers and aedamers.
These are of particular current interest because of their applicabil-
ity in emulating the dynamic structural characteristics of natural
Amongst other evidence, the interactions between porphyrin
and diimide was characterised by upfield shifts in the H NMR
1
spectrum of both the diimide and the inner porphyrin NH protons
as a result of the mutual shielding of the aromatic rings in a
face-to-face conformation. This implies an inherent and strong
p–p interaction, possibly involving a charge-transfer, between the
electron-poor diimide subunit and the electron-rich porphyrin.
In this paper, we investigate the generality of this POR–NDI
porphyrin–diimide interaction, and explore how it can be utilised
in the formation of foldamers for specific multi-functional systems
that incorporate these units.
Results and discussion
Chemistry, School of Science and Technology, University of New England,
Armidale, NSW, 2351, Australia. E-mail: mgunter@une.edu.au; Fax: 612-
6773-2368; Tel: 612-6773-2767
Synthesis
† Electronic supplementary information (ESI) available: 1H NMR spectra
and associated numbering schemes for major compounds discussed in the
text; selected variable temperature NMR spectra; van’t Hoff plot of 3 with
DN38C10. See DOI: 10.1039/b804267e
Since the POR–NDI interaction appeared to be significant and
general, we devised a series of structures that would enable a
more detailed study of the phenomenon. In the first instance,
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the pyridine-terminated porphyrin diimide 1a (Fig. 1), previously
reported by us,16 provided an opportunity to investigate the
effects of intramolecular coordination on the stacking of the
aromatic units. Subsequently, to ascertain if the pyridine unit is
a factor in the stacking interaction, we synthesised a structural
modification of 1a in which the pyridine unit is replaced by a simple
ester, 3 (Fig. 3). At the same time, we changed the attachment
of the linking polyoxoethylene chain to an amide at the para
position of the meso aromatic group of the porphyrin, 2 (Fig. 3),
which effectively requires a longer span of the linker to achieve
an intramolecularly stacked structure. Further, to promote the
possibility of intramolecular foldamers, the ethyl esters of 2 and
3 were replaced with a second porphyrin in each case, resulting in
10 and 11 (Fig. 8). Replacement of the amide linkages of 10 with
a less constrained secondary amine linker in 12 (Fig. 8) completed
the bis-porphyrin series. The non-porphyrinic analogues 15 and 16
(Fig. 10) with varying p-acidity of the aromatic groups, provided
a test of the prerequisite of the porphyrin unit in the stacking
motif. Finally, shorter chain homologues of 3 and 11, viz. 18 and
17 (Fig. 11), with diethyleneoxy linkages in place of the longer
tetraethylenoxy chains, provided the possibility of more tightly
constrained foldamers.
resonance is normally shifted upfield by shielding from the crown
aromatics to around 8.2–8.3 ppm.2,7 By contrast, the diimide
resonance of 1a appears at 7.2 ppm, and shifts upfield with
decreasing temperature, but is concentration-independent. This
is consistent with an intramolecular folded structure as depicted
in 1a.
Zinc was inserted into the diimide-fused porphyrin producing
1b (Fig. 1), to establish whether this might disrupt the porphyrin–
diimide interaction: the terminal pyridine of 1b was expected
to coordinate intramolecularly to the zinc porphyrin and thus
interrupt the porphyrin–diimide interaction. However, proton
NMR analysis of the compound showed diimide upfield shifts
indicative of a porphyrin–diimide interaction being maintained,
in addition to the terminal pyridyl–zinc coordination. This
suggests the structure shown in Fig. 2, with the long tetraethylene
glycol-derived linker groups allowing both the porphyrin–diimide
interaction on one face of the porphyrin and the pyridine–zinc
coordination on the other.
Fig. 1
The synthetic route of these derivatives was based on similar
strategies used for the production of rotaxane structures previously
reported by us.15 The ethyl esters 2 and 3 (Fig 3) and their
corresponding bis-porphyrin analogues 10 and 11 (Fig. 8) were
produced in a single reaction by an EDC condensation of
the corresponding porphyrins 14 and 13 (Fig. 8) with the bis
monosuccinate ester 4 (Fig. 3) of the dihydroxy polyoxoethylene
naphthodiimide 5 (Fig. 6). In each case, the presence of ethanol
in the reaction mixture gave directly separable mixtures of 2
and 10, or 3 and 11. A similar strategy produced the shorter-
chain derivatives 17 and 18 (Fig. 11). The secondary amine
derived structure 12 (Fig. 8) was produced by condensation of
5 with 4-carboxybenzaldehyde, subsequent titanium tetrachloride
catalysed formation of the Schiff base with the zinc derivative of
porphyrin 14 and in situ reduction with cyanoborohydride.
Fig. 2 Above: the folding effect expected‡ for the zinc metallated
derivative 1b, incorporating both pyridine–zinc coordination and di-
imide–porphyrin interaction. Two of the linker ethyleneoxy groups from
each side of the diimide sub-unit have been omitted for clarity. Below:
the expected conformation of 1c, the ruthenium(II) metallated derivative
of 1a. The terminal pyridine coordinates to the central ruthenium(II)
ion, while the carbonyl ligand on the alternate porphyrin face prevents
diimide–porphyrin stacking interactions.
Conformational studies
Pyridine-terminated diimide porphyrin 1a. Typically, the pro-
ton resonances in their 1H NMR spectra of various alkyl
and polyethylene glycol-substituted naphthodiimide open-chain
derivatives appear around 8.7 ppm; for naphthodiimide-derived
The simple modification of 1a, by utilising a six-coordinate
ruthenium(II) with its carbonyl ligand as the central porphyrin
metal ion in the place of zinc, was expected to prevent such
catenanes where the diimide unit is threaded through
a
1,5-dinaphtho-38-crown-10 (DN38C10) macrocyclic unit, this
‡ This depiction is suggested, and is not an energy minimised structure.
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a folding effect, as the carbonyl would block the face of the
porphyrin opposite to the pyridyl coordinating face. Proton NMR
analysis of the resulting structure 1c (Fig. 2), previously reported
by us,16 showed the diimide peak in its unshielded (d 8.7) posi-
tion, suggesting successful prevention of the porphyrin–diimide
interaction and the structural conformation shown in Fig. 1.
the presence of intramolecular POR–NDI interactions similar to
those seen in 1a (ESI, Fig. S1 and S2†). In these cases, because of
the molecular asymmetry, the diimide proton signals now appear
as AB quartets, but at 7.76–7.88 ppm for 2 and 7.19–7.29 ppm for 3,
still significantly shielded compared to typical unassociated ‘free’
diimides; the porphyrin core NH protons were also still shielded
at −3.38 ppm for 2 and −3.84 ppm for 3.
Mono-porphyrin linked diimides. In the case of the mono-
porphyrin diimide long chain units 2 and 3 (Fig. 3) with ethyl
esters replacing the pyridine unit of 1a, the NMR spectra revealed
Since these porphyrin mono-stoppered derivatives are capable
of threading through a DN38C10 macrocycle, the formation
of the resultant pseudorotaxane should be accompanied by a
release of the folded structure to accommodate the macrocycle.
Indeed, the spectrum of a 1 : 1 mixture of 3 and DN38C10 in
deuterochloroform showed peaks for both bound and unbound
species over a range of low temperatures where the system was in
slow exchange (<270 K).§ In this case, the NDI proton signal was
observable in both its unbound (∼7.2 ppm) and bound (∼8.2 ppm)
positions. The bound position was identical to that for the
pseudorotaxane formed from the non-porphyrinic dicarboxylic
acid precursor 4 and DN38C10, and this peak grew at the expense
of that for the unbound (but folded) species at 7.2 ppm. This
is consistent with the formation of the pseudorotaxane structure
[DN38C10.3] (Fig. 4) in which the encircling crown ether now
precludes the foldamer conformation, as expected.
From this data, the binding strength of the DN38C10 complex
with 3 was calculated to be 3.98 × 102 M−1 at 243 K. This compares
to the Ka of the complex between 4 and DN38C10 at the same
temperature of 3.52 × 103 M−1, and indicates that while DN38C10
binds to 3 less strongly, it adopts a similar open conformation
when bound. A van’t Hoff plot of this system gave a non-linear
relationship between R ln K and 1/T (see ESI, Fig. S3†), as did
the [DN38C10.1a] system, which is indicative of a conformational
or other change accompanying the binding process (i.e. in these
cases, an unravelling of the foldamer).¶
§ At higher temperatures, in the fast exchange regime, only a temperature-
dependent time-averaged peak is seen for the diimide resonance.
¶ The reason for the weaker complexation is unclear, as intuitively one
might expect a stronger interaction with the porphyrin-stoppered adduct,
as this has the possibility of an alternative site for a p-stacked complex,
now between the DN38C10 aromatics and the porphyrin ring. We had
previously reported15 a non-linear van’t Hoff plot for the system involving
DN38C10 and 1c in the formation of a pseudo-catenane, with a Ka of
140 M−1 at 273 K, and a non-zero heat capacity term DCp as a result of
an unfolding required for the formation of the pseudo-catenane. This is
similar to the results seen for the free base derivative 1a.
Fig. 3
Fig. 4
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The porphyrin–diimide orientation in these systems is expected
to be similar to the calculated structure shown in Fig. 5, with
the diimide lying face-to-face with the porphyrin to optimise the
p–p interactions between the two aromatics. The space-filling
model also shown in Fig. 5 illustrates that the diimide appears
to have an ideal size and shape to match the cavity afforded by
the meso-aromatics on the porphyrin face. It is likely that it is a
combination of the p–p interactions and the snug fitting nature
of the POR–NDI porphyrin–diimide interaction that makes it
so significant, and if so, then it is expected that relatively stable
complexes should be maintained intermolecularly for un-linked
porphyrins and diimides.
Fig. 6
intermolecular interaction. The association constant for 5 with 6
was calculated as 15.6 1.2 M−1 from NMR titration experiments.
It is expected that a conformation as depicted in Fig. 5 would
be destabilised by any porphyrin with sterically demanding groups
on each of its faces. Thus, a series of porphyrins, including 6, 7, 8
and 9 (Fig. 6) were analysed using NMR methods. 9, with its bulky
methoxy ortho substituents, was specifically designed to block any
close approach of the diimide and porphyrin. The other porphyrins
contained either two or four meso- aromatics with para or meta
substituents.
When individually mixed with 5 in CDCl3, 6, 7 and 8 each
exhibited significant diimide shifts, indicative of the expected
diimide–porphyrin interaction. However, the bulky methoxy ortho
substituted porphyrin 9 showed only an unshielded diimide peak at
d 8.7, suggesting a blocking of the diimide–porphyrin interaction
through steric hindrance. These results were duplicated using a
pyromellitimide thread in place of the naphthalene diimide, with
the same series of porphyrins.
UV analysis confirmed this result: difference absorption spec-
tra were measured between the porphyrin–diimide systems and
equimolar solutions of the respective porphyrin (Fig. 7). Since the
diimide 5 exhibits no absorption above 400 nm, any peaks observed
must arise from new absorbances caused by the interaction
between the diimide and porphyrin, specifically charge-transfer
bands. The mixtures of 5 with 6, and 5 with 7 exhibited a new
band that can be attributed to a charge transfer absorption at
432 nm, while the corresponding band for the mixture of 5
with 8 appeared at 426 nm. The sterically-hindered porphyrin
9 showed no charge transfer absorbance in the examined spectral
region. These results are consistent with the corresponding NMR
findings that 9, with its sterically-hindering ortho substitution, is
prevented from forming a stable complex. Similar charge-transfer
absorption results were found using pyromellitimide in place of
the naphthalene diimide thread.
Fig. 5 Depiction of the calculated* diimide–porphyrin interaction in 1a:
top view, with the diimide lying above the porphyrin face (top); and
space-filling depiction (bottom). Two of the linker ethyleneoxy groups
were omitted from the linked side of the diimide sub-unit and only an
ethyl group was included on the other (free) side of the diimide to simplify
the calculation.
Intermolecular systems. This was validated by the fact that
an equimolar mixture of tetratolyl porphyrin 6 with naphthalene
diimide 54 (Fig. 6) in CDCl3 produced diimide and porphyrin
N–H proton NMR upfield shifts similar to 1a, indicating an
* AM1 semiempirical energy minimisation calculation performed using
the MacSPARTAN program. The porphyrin with its meso-substituted
aromatics was minimised separately first, then this component of the
structure was locked, while the diimide thread component was allowed
to move. The resultant structure shown is the calculated lowest energy
position for the diimide.
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Fig. 7 UV–vis spectra in CHCl3 of: 5, 6, 7, 8 and 9 (top); and difference
spectra of equimolar mixtures of 5 with: 6, 7, 8 and 9 (bottom), measured
against the respective porphyrin component.
Bis-linked porphyrin diimide foldamers. This inherent capacity
for face-to-face p-stacking between facially unhindered meso-
substituted porphyrins and naphthodiimides is also evident in
other linked systems which can adopt folded conformations in
solution, similar to that seen in 1, 2, and 3.
1
This can be seen for example in H NMR spectra of the three
bis-porphyrin adducts 10, 11 and 12 (Fig. 8), all of which showed
significant upfield shifts of the diimide proton resonances (7.42,
6.93 and 7.11 ppm in CDCl3 for 10, 11 and 12, respectively),
indicative of a solution conformation involving a shielding of the
diimide unit, either by an inter- or intra-molecular interaction;
an S-shaped foldamer POR–NDI–POR as depicted in Fig. 8 is
suggested (see ESI, Fig. S4, S5, S6†).
Complementary shifts of the inner NH protons of the porphyrin
(Dd −0.50, −0.68 and −0.59 ppm respectively in each case,
compared to the unattached porphyrin precursors 13 or 14) are
consistent with such a foldamer conformation with face-to-face
stacking of the diimide and porphyrin units. The folding of the
polyoxyethylene chains inherent in the S-shaped foldamer was also
apparent by NOESY correlations between the protons adjacent
to the diimide N, and the succinyl ester group protons in 11.
Concentration-invariance of the spectrum is consistent with a
foldamer, rather than possible alternative intermolecular stacking.
At higher temperatures, the diimide and inner porphyrin NH
resonances gradually shift downfield in tandem, as p–p stacking is
diminished (Dd +0.28 between 303 K and 328 K for 10 in CDCl3);
there is minimal change in other resonances (see ESI, Fig. S7†).
Similar temperature dependence was observed for the ester-linked
11 and the amine-linked 12.
Fig. 8
For the meta-linked derivative 11, the mutual shielding of the
porphyrin and diimide units is even more pronounced than in the
para-substituted amide analogue 10. The diimide protons of 11
are upfield shifted by −1.80 ppm, compared to −1.31 ppm for 10,
and the porphyrin NH’s are also more shielded (Dd −0.68 ppm
compared to −0.50 ppm). In this case, because of the absence
of the plane of symmetry through the porphyrin unit, the ortho-
protons of the 3,5-di-tert-butyl groups on the meso-substituents
appeared as closely coupled separated resonances. Significantly,
only one of these resonances showed ROESY correlations with
the diimide protons, obviously the ortho-protons on the face of
the porphyrin proximal to the diimide in the foldamer structure.
Further evidence for the S-shaped foldamer conformation
was derived from metallation of the porphyrin. Thus, in the
corresponding zinc derivative of 10 it was confirmed that there
was little effect on the diimide–porphyrin interaction in the
non-coordinating CDCl3 solvent. Indeed, the diimide resonance
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now appeared at even higher field (d 7.19 ppm), indicating an
enhanced interaction. This may be purely electronic in origin (less
electronegative zinc derivative compared to free base), or perhaps
due to an augmentation by appropriate coordination of the zinc
ion to one of the O atoms in the polyether chain. Shifts in many of
the polyether methylene resonances favour the latter. Nevertheless,
since zinc porphyrins readily form 5-coordinate complexes in
the presence of nitrogenous bases, the addition of pyridine may
cause unfolding of the bis-adduct by displacing the diimide; on
the other hand, axial coordination at the “outside” faces of the
zinc porphyrins would have little effect on the diimide–porphyrin
stacking, as depicted in Fig. 9B.
Further evidence for the POR–NDI–POR foldamer structure
resulting from p–p interactions was provided by the H NMR
1
spectrum of 10 in benzene-d6. In this solvent, the pyrrolic NH
protons were now shifted downfield to −2.74 ppm (Dd +0.45 ppm
relative to the spectrum in CDCl3) (see ESI, Fig. S9†). This is a
similar chemical shift to that for the previously reported15 [2]ro-
taxane comprising 10 threaded through the dinaphtho-38-crown-
10 macrocycle, [DN38C10.10]. In this rotaxane, the NDI unit is
sandwiched between the aromatic groups of the crown ether, which
effectively prevents any POR–NDI interactions. By analogy then,
in this case, this is consistent with an unfolded conformation
with the POR–NDI interactions replaced by benzene–NDI p–p
stacking, as indicated in Fig. 9C.
The benzene–diimide interactions in this solvent also impose an
enhanced upfield shift on the naphthodiimide protons, which now
appear at 6.85 ppm. The fact that these protons are shifted further
upfield than in CDCl3 can be rationalised by (i) a more effective
overlap of the smaller benzene p-system compared to larger, more
diffuse, and peripherally crowded porphyrin rings; and (ii) an
equilibrium effect shifted towards more complete intermolecular
complexation as a result of the larger effective molarity of the
p-donor constituent in the neat d6-benzene solvent.
Although zinc insertion into the porphyrin did not interfere
with the S-shaped POR–NDI–POR foldamer conformation, pro-
tonation of the free base porphyrin was effective in destroying
the stabilising interactions, leading to unfolding. This was clearly
seen by the downfield shift of the diimide proton signal in the
NMR spectrum of 10 in CDCl3 containing trifluoroacetic acid, to
8.72 ppm, a chemical shift typical of ‘free’ naphthodiimides in the
absence of inter- or intra-molecular p-stacking effects (see ESI,
Fig. S10†). Protonation of free base porphyrins is accompanied
by a ruffling of the planar structure,23 which apparently in this
case is sufficient to introduce steric constraints to any stacked
conformations of the porphyrin and diimide aromatic systems.
In this instance, the NH protons in the porphyrin core were no
longer diagnostic, as they appeared upfield shifted as expected
for a protonated porphyrin, as were the b-pyrrolic and other
porphyrinic aromatic resonances.
Fig. 9 Depiction of the folded and extended conformations in the
bis-porphyrin diimide derivatives 10, 11, and 12 as deduced from NMR
measurements. The dashed lines indicate co-facial p–p interactions.
The p-stacking interactions responsible for the foldamer struc-
ture in the POR–NDI–POR systems are not maintained if the
porphyrin units are replaced by smaller aromatic groups. The non-
porphyrinic analogues 15 and 16 (Fig. 10) where the porphyrin has
Thus, on increasing addition of pyridine to a CDCl3 solution
of Zn10, as well as small expected shifts in some of the porphyrin
resonances, broadened zinc-coordinated pyridine resonances ap-
peared, in a pattern and position typical of pyridine-coordinated
zinc porphyrins (for these complexes, ligand exchange is fast on
the NMR chemical shift timescale, so the ligand resonances are
broadened and shifted upfield and gradually move downfield
with increasing pyridine addition). After two mole equivalents
of pyridine per zinc porphyrin, the diimide resonance had moved
slightly downfield to a limiting position of 7.45 ppm, still well short
of the value of 8.7 ppm expected for a non-interacting, unfolded
conformation. At the same time, the previously shifted polyether
methylene signals now moved back to their approximate position
as in 10, consistent with a replacement of the presumed polyether
oxygen coordination to the zinc by the stronger pyridine ligand
(see ESI, Fig. S8†). This is then compatible with a structure with
“outside” coordination of the pyridine, as indicated in Fig. 9B.
Fig. 10
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been replaced by either p-tolyl or 2,4-dinitrophenyl groups show
no evidence of shielding of the naphthodiimide proton signals
in solution, and they appear in the ‘normal’ range, viz. 8.67
and 8.72 ppm, respectively, indicative of an extended, unfolded
conformation.
The folded conformation is also maintained with the shorter
dioxyethylene linked derivatives 17 and 18 (Fig. 11), as evidenced
by: the upfield shifts of both diimide (Dd −2.09 and −1.88 ppm
respectively) and porphyrin NH (Dd −0.79 and −1.41 ppm, respec-
tively) resonances in their NMR spectra; a NOESY correlation
between the diimide and the downfield shifted (deshielded by the
diimide) ortho1 protons of the meso aromatic groups, as well as
the b-pyrrolic protons; and an enhanced deshielding (compared
to the longer chain derivative 11) of the methylene signals in the
dioxyethylene chains as a result of stronger deshielding by the
diimide and porphyrin rings in this more tightly folded structure.
Fig. 12
correlations between the propynyl methylene and the b-methyl
and ethyl resonances in more concentrated solutions are further
evidence of an intermolecular stacking between diimide and
porphyrin units, as the relatively constrained geometry of 19 would
render intramolecular correlations unlikely. This interaction is not
possible in the rotaxane consisting of 19 as the thread unit through
a DN38C10 macrocycle, [DN38C10.19], as we have reported
previously,15 and the NMR shifts are as expected for these types
of rotaxanes.
Experimental
In the following descriptions, NMR assignments follow the
numbering schemes indicated for the respective compounds in
the ESI.†
2-(2-(2-(2-(2-(5,10,15-Tris((3,5-di-tert-butyl)phenyl)-20-(4-(3-car-
boxypropionamido)phenyl)porphyrinyloxy)ethoxy)ethoxy)ethoxy)-
ethyl)-7-(2-(2-(2-(2-(3-ethoxycarbonyloxy)propionyloxy)ethoxy)-
ethoxy)ethoxy)ethyl)benzo[lmn]-(3,8)phenanthroline-1,3,6,8-
tetraone 2 and 2,7-bis-(2-(2-(2-(2-(5,10,15-tris((3,5-di-tert-
butyl)phenyl)-20-(4-(3-carboxypropionamido)phenyl)-
porphyrinyloxy)ethoxy)ethoxy)ethoxy)ethyl)benzo[lmn]-
(3,8)phenanthroline-1,3,6,8-tetraone 10
To a solution of 2,7-bis-(2-(2-(2-(2-(3-carboxypropionyloxy)eth-
oxy)ethoxy)ethoxy)ethyl)benzo[lmn]-(3,8)phenanthroline-1,3,6,8-
tetraone 415 (25 mg, 3.05 × 10−5 mol) in freshly distilled CHCl3
(14 mL) containing EtOH (15 ll) were sequentially added
EDC (14.6 mg, 7.63 × 10−5 mol), 1-hydroxybenzotriazole,
HOBT (16.5 mg, 1.22 × 10−4 mol) and 5,10,15-tris((3,5-di-tert-
butyl)phenyl)-20-(4-aminophenyl)porphyrin24 (62 mg, 6.41 ×
10−5 mol). Dry Et3N (14 ll, 1.01 × 10−4 mol) was then added
and the reaction was stirred for 7 d (N2, dark). Excess solvent
was removed in vacuo, and the crude mixture was washed with
dilute HCl (2 M), water, saturated NaHCO3, water (3×) and
taken up in DCM. The organic layer was then dried (Na2SO4)
and evaporated in vacuo. Purification was carried out using
preparative TLC silica plates with 5% MeOH–DCM as the eluant
to afford unreacted starting porphyrin (14 mg, 22.5% recovered),
Fig. 11
On the other hand, it is obvious that the more rigidly con-
strained bis-porphyrin diimide 19 (Fig. 12), which has previously
been incorporated as the thread component of a [2]rotaxane,15
cannot form the S-shaped foldamer structure of the more flexible
systems described above. Nevertheless, the diimide resonances
of this molecule appear upfield from the ‘normal’ range at
7.45 ppm, with the porphyrin NH’s also somewhat upfield at
−2.95 ppm.
However, in this case, the shifts are concentration-dependent,
indicating an inter-molecular stacking pattern. NOESY
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bis-stoppered porphyrin thread 10, and mono-stoppered thread
2:
7.82 (3H, s, ortho2), 7.79 (3H, s, para), 7.65 (1H, t, J 6, H23), 7.50
(1H, d, J 6, H24), 7.29 (2H, d, J 9, ND2), 7.19 (2H, d, J 6, ND1),
4.15 (2H, t, J 6, H26), 4.11 (2H, t, J 6, H33), 4.08 (4H, m, H45,
46), 4.01 (2H, t, J 6, H38), 3.59 (2H, t, J 6, H27), 3.53 (10H, m,
H28–32), 3.50–3.46 (12H, m, H39–44), 3.08 (2H, t, J 6, H35), 2.91
(2H, t, J 6, H34), 2.59 (4H, t, J 3, H36, 37), 1.66 (27H, s, t-butyl1),
1.46 (27H, s, t-butyl2), 1.24 (3H, t, J 6, H47), −3.84 (2H, s, H25).
10 (11.8 mg, 14%) as a purple solid (from MeOH–CHCl3); mp
184–187◦C; m/z (ES-MS) (C174H200N12O16): [M + 2H]2+ 1358.774
1
(calc. 1358.771); H NMR (300 MHz, CDCl3) d 8.87 (16H, m,
H2, 3, 7, 8, 12, 13, 17, 18), 8.17 (4H, d, J 6, H22), 8.17–8.15
(2H, hidden, H23), 8.12 (12H, ms, ortho), 7.90 (4H, d, J 9,
H21), 7.79 (6H, ms, para), 7.42 (4H, s, ND), 4.30 (4H, t, J 6,
H26), 4.05 (4H, t, J 6, H33), 3.70 (4H, t, J 6, H27), 3.60 (12H,
m, H28–30), 3.52 (8H, m, H31–32), 2.85 (4H, d, J 6, H34),
2.76 (4H, d, J 6, H35), 1.53 (108H, s, t-butyl), −3.19 (4H, s,
H25).
2,7-Bis(2-(2-(2-(2-(4-formylphenylcarboxy)-ethoxy)-ethoxy)-
ethoxy)-ethyl)benzo[lmn]-[3,8]phenanthroline-1,3,6,8-tetraone 20
This was synthesised by an analogous method to that used for
2, using dihydroxydiimide thread 5 (0.101 g, 1.64 × 10−4 mol),
4-carboxybenzaldehyde (0.079 g, 5.26 × 10−4 mol), dry DCM
(10 mL), dry Et3N (76 ll, 5.71 × 10−4 mol), EDC (0.160 g, 8.35 ×
10−4 mol), HOBT (0.088 g, 6.51 × 10−4 mol) and stirring for 7 d.
Upon removal of excess solvent, a yellow oil (144 mg, 99.5%) was
obtained, which was sufficiently pure for further reaction. (ESI-
MS) (C46H46N2O16): [M + Na]+ 905.276 (calcd. 905.275); 1H NMR
(300 MHz, CDCl3) d 10.02 (2H, s, H48), 8.65 (4H, s, ND), 8.13
(4H, d, J 9, H34), 7.88 (4H, d, J 9, H35), 4.43–4.36 (8H, m, H26,
33), 3.80 (8H, m, H27, 28), 3.65–3.64 (4H, m, H30), 3.63–3.56
(12H, m, H29, 31, 32).
2 (2.7 mg, 5%) as a purple solid (from MeOH–CHCl3); mp
100–104◦C; m/z (ESI-MS) (C108H127N7O17): [M + H]+ 1794.923
1
(calc. 1794.937); H NMR (300 MHz, CDCl3) d 8.85–8.80 (8H,
m, H2, 3, 7, 8, 12, 13, 17, 18), 8.26 (1H, br, H23), 8.20 (2H, d,
J 9, H22), 8.14–8.12 (6H, m, ortho), 7.95 (2H, d, J 9, H21), 7.88
(2H, d, J 8, ND2), 7.79–7.77 (3H, m, para), 7.76 (2H, d, J 8,
ND1), 4.38 (2H, t, J 5, H26), 4.23 (2H, t, J 6, H33), 4.17–4.11
(4H, m, H45, 46), 4.09 (2H, t, J 7, H38), 3.78 (2H, t, J 5, H27),
3.70–3.64 (10H, m, H28–32), 3.59–3.49 (12H, m, H39–44), 2.89
(2H, t, J 6, H34), 2.85 (2H, t, J 6, H35), 2.58–2.56 (4H, m, H36,
37), 1.54 (54H, s, t-butyl), 1.23 (3H, t, J 6, H47), −3.38 (2H, s,
H25).
5,10,15-Tris((3,5-di-tert-butyl)phenyl)-20-(4-
aminophenyl)porphyrinato)zinc(II) 21
2-(2-(2-(2-(2-(5,10,15-Tris((3,5-di-tert-butyl)phenyl)-20-(3-(3-car-
boxypropionoyl)phenyl)porphyrinyloxy)ethoxy)ethoxy)ethoxy)-
ethyl)-7-(2-(2-(2-(2-(3-ethoxycarbonyloxy)propionyloxy)ethoxy)-
ethoxy)ethoxy)ethyl)benzo[lmn]-(3,8)phenanthroline-1,3,6,8-
tetraone 3 and 2,7-bis-(2-(2-(2-(2-(5,10,15-tris((3,5-di-tert-
butyl)phenyl)-20-(3-(3-carboxypropionoyl)phenyl)porphyrinyloxy)-
ethoxy)ethoxy)ethoxy)ethyl)benzo[lmn]-(3,8)phenanthroline-
1,3,6,8-tetraone 11
Zinc was inserted into the free-base amino phenyl porphyrin using
the standard method (Zn(OAc)2–DCM–MeOH) t◦o give 21 as a
1
reddish purple solid (73 mg 98%); mp 299–302 C; H NMR
(300 MHz, CDCl3) d 8.96–8.91 (6H, m, H3, 7, 8, 12, 13, 17), 8.78
(2H, d, J 5, H2, 18), 8.11 (2H, d, J 2, ortho), 8.06 (4H, d, J 2,
ortho), 7.76 (1H, t, J 2, para), 7.72 (2H, t, J 2, para), 7.62 (2H, d,
J 8, H21), 5.63 (2H, br, H22), 1.50 (27H, s, t-butyl), 1.41 (27H, s,
t-butyl), −1.35 (1H, br, H23).
This was synthesised by a method similar to that described above
for 2, using 2,7-bis-(2-(2-(2-(2-(3-carboxypropionyloxy)eth-
oxy)ethoxy)ethoxy)ethyl)benzo[lmn]-(3,8)phenanthroline-1,3,6,8-
tetraone (23 mg, 2.81 × 10−5 mol), dry CHCl3 (13 mL), EtOH
(15 ll) dry Et3N (13 ll), EDC (13.5 mg, 7.04 × 10−5 mol),
HOBT (7.6 mg, 5.62 × 10−5 mol) and 5,10,15-tris((3,5-di-
tert-butyl)phenyl)-20-(3-hydroxyphenyl)porphyrin15 (60 mg,
6.20 × 10−5 mol). The crude product was purified by silica column
chromatography (1% MeOH–CHCl3) to give unreacted porphyrin
(10.5 mg, 17.5%), the bis-porphyrin thread unit 11 and 3:
2,7-Bis(2-(2-(2-(2-(4-(5-(phen-4-yl)10,15,20-tris((3,5-di-tert-
butyl)phenyl)porphyrinyl)-aminophenyl-4-benzylcarboxy)-ethoxy)-
ethoxy)-ethoxy)-ethyl)benzo[lmn]-[3,8]phenanthroline-1,3,6,8-
tetraone 12
Following the modified method outlined by Barney et al,25 to
a solution of naphthodiimide dialdehyde 20 (21.4 mg, 2.43 ×
10−5 mol), dry DCM (50 mL), and dry Et3N (10 ll, 7.17 × 10−5 mol)
was added NaBH3CN (4.6 mg, 7.29 × 10−5 mol). The solution
immediately went bright red, which was strongly indicative of
charge transfer complex formation. The mixture was equilibrated
for 60 min at RT after which time zinc amino phenyl porphyrin
21 (50 mg, 4.86 × 10−5 mol) and a solution of TiCl4 in DCM
(12.50 ll, 1.25 × 10−5 mol) were added. The reaction was then
stirred for 18 h before it was quenched with a THF solution of
NaBH3CN and the stirring was continued for a further 6 h. The
reaction mixture was then washed with 5 M NaOH, water (3×)
and dried (Na2SO4). Excess solvent was removed in vacuo. Multiple
purifications on preparative TLC silica plates (2% MeOH–DCM)
afforded the free-base derivative of amino phenyl porphyrin 21
(14 mg, 29.0% recovered), bis adduct 12 (5.8 mg, 4.3%), and the
corresponding mono-porphyrin adduct (16.8 mg, 14%).
11 as a purple solid (from MeOH–CHCl3) (17.2 mg, 23%); mp
182–184 ◦C; m/z (ESI-MS) (C174H198N10O18): [M + 2H]2+ 1359.759
(calc. 1359.755); 1H NMR (300 MHz, CDCl3) d 8.86–8.81 (16H,
m, H2, 3, 7, 8, 12, 13, 17, 18), 8.34 (6H, s, ortho1), 8.23 (2H, d, J
3, H21), 7.94 (2H, d, J 6, H22), 7.92 (6H, s, ortho2), 7.80 (6H, s,
para), 7.67 (2H, t, J 6, H23), 7.52 (2H, d, J 6, H24), 6.93 (4H, s,
ND), 4.22 (4H, t, J 6, H26), 4.00 (4H, t, J 6, H33), 3.61 (4H, t, J
6, H27), 3.48 (12H, m, H28–30), 3.43 (8H, m, H31–32), 3.01 (4H,
t, J 6, H35), 2.84 (4H, t, J 6, H34), 1.60 (54H, s, t-butyl1), 1.48
(54H, s, t-butyl2), −3.40 (4H, s, H25).
3 (3.3 mg, 6.6%) as a purple solid (from MeOH–CHCl3); mp
95–99 ◦C; m/z (ES-MS) (C108H126N6O18): [M + Na]+ 1817.904
1
(calc. 1817.903), [M + H]+ 1795.922 (calcd. 1795.921); H NMR
(300 MHz, CDCl3) d 8.85–8.79 (8H, m, H2, 3, 7, 8, 12, 13, 17,
18), 8.53 (3H, s, ortho1), 8.41 (1H, s, H21), 7.83 (1H, d, J 6, H22),
12 (from CHCl3–MeOH) mp 211–215 ◦C; m/z (ES-MS)
(C182H204N12O14): [M + Li]+ 2789 (calcd. 2788.56), [M + 2H]2+
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1392.793 (calcd. 1392.792); 1H NMR (300 MHz, CDCl3) d 8.87–
8.80 (16H, m, H2, 3, 7, 8, 12, 13, 17, 18), 8.12 (12H, s, ortho),
8.08 (4H, d, J 9, H34), 8.02 (4H, d, J 9, H21), 7.78 (6H, s,
para), 7.57 (4H, d, J 9, H35), 7.11 (4H, s, ND), 6.93 (4H, d,
J 9, H22), 4.60 (4H, s, H46), 4.49 (2H, br, H23), 4.44 (4H, t,
J 6, H26), 4.01 (4H, t, J 6, H33), 3.79 (4H, t, J 6, H27), 3.63
(4H, t, J 3, H30), 3.58 (4H, t, J 3, H31), 3.52 (8H, m, H28,
29), 3.44 (4H, t, J 6, H32), 1.53 (108H, s, t-butyl), −3.28 (4H, s,
H25).
Na]+ 665.161 (calcd. 665.159); 1H NMR (300 MHz, CDCl3) d 8.76
(4H, s, ND), 4.48 (4H, t, J 5, H26), 4.18 (4H, t, J 6, H29), 3.89
(4H, t, J 5, H27), 3.72 (4H, t, J 4, H28), 2.30 (8H, s, H34, 35).
2,7-Bis(2-(2-(3,5-(phen-3-yl)10,15,20-tris((3,5-di-tert-butyl)phe-
nyl)porphyrinyl)carboxypropionyloxy)-ethoxy)-ethyl)benzo[lmn]-
[3,8]phenanthroline-1,3,6,8-tetraone 17 and 2-((2-(2-(3,5-(phen-
3-yl)10,15,20-tris((3,5-di-tert-butyl)phenyl)porphyrinyl)carboxy-
propionyloxy))-7-[(2-(2-(3-carboxypropionyloxy)]-ethoxy)-
ethyl)benzo[lmn]-[3,8]phenanthroline-1,3,6,8-tetraone 18
2,7-Bis(2-(2-(2-(2-(4-(phen-4-yl)methyl)carboxamido-
propionyloxy)-ethoxy)-ethoxy)-ethoxy)-ethyl)benzo[lmn]-
[3,8]phenanthroline-1,3,6,8-tetraone 15
Synthesised as for 2, using the dicarboxylic acid axle 22 (15 mg,
2.35 × 10−5 mol), freshly distilled CHCl3 (12 mL), EtOH (15 lL),
dry Et3N (19.6 mg, 1.94 × 10−4), EDC (39.6 mg, 2.07 × 10−4 mol),
HOBT (13.0 mg, 0.96 × 10−−4 mol) and hydroxy phenyl porphyrin
(50 mg, 5.16 × 10−5 mol). After 3 d (N2, dark), excess EDC
was added and the reaction was then stirred for a further 1 d.
An initial purification was carried out on preparative TLC silica
plates using 50% petroleum spirit–CHCl3 as the eluant before a
final purification was carried out (preparative TLC silica plates)
using 30% petroleum spirit–CHCl3 as the eluant to obtain the bis
porphyrin stoppered adduct 17 and mono porphyrin stoppered
adduct 18.
This was synthesised by a similar procedure to that used for 2,
using the dicarboxylic acid thread (28 mg, 3.42 × 10−5 mol), EDC
(16.3 mg, 8.55 × 10−5 mol), HOBT (18.5 mg, 1.37 × 10−4 mol),
4-toluidine (7.3 mg, 6.84 × 10−5 mol) and dry DCM (23 mL).
The mixture was stirred at RT for 3 d. Purification was carried
out on a preparative TLC silica plate using 5% MeOH–DCM as
1
the eluant to obtain 15 as a yellow oil (8.5 mg, 25%); H NMR
(300 MHz, CDCl3) d 8.67 (4H, s, ND), 7.77 (2H, br, H23), 7.30
(4H, d, J 8, Hc), 6.98 (4H, d, J 8, Hb), 4.42 (4H, t, J 5, H26),
4.19 (4H, t, J 5, H33), 3.83 (4H, t, J 5, H27), 3.69–3.50 (20H, m,
H28–32), 2.73 (4H, d, J 6, H35), 2.62 (4H, d, J 6, H34), 2.22 (6H, s,
Ha).
17 as a purple solid (from MeOH–CHCl3) (15.0 mg, 25%); mp
238–241 ◦C; m/z (ES-MS) (C166H182N10O14): [M + 2H]2+ 1271.703
1
(calcd. 1271.699); H NMR (300 MHz, CDCl3) d 8.84 (16H, m,
H2, 3, 7, 8, 12, 13, 17, 18), 8.41 (6H, s, ortho1) and (2H, peak
hidden underneath, H21), 7.89 (6H, s, ortho2), 7.89–7.85 (2H, d,
H22), 7.79 (6H, m, para), 7.64 (2H, t, H23), 7.48 (2H, d, H24),
6.67 (4H, s, ND), 4.12 (4H, t, J 6, H26), 3.96 (4H, t, J 6, H29),
3.57 (4H, t, J 6, H27), 3.38 (4H, t, J 6, H28), 2.98 (4H, t, J 6,
H35), 2.78 (4H, t, J 6, H34), 1.62 (54H, s, t-butyl1), 1.47 (54H, s,
t-butyl2), −3.51 (4H, s, H25).
2,7-Bis(2-(2-(2-(2-(4-(dinitrophen-2,4-yl)10,15,20-tris((3,5-di-tert-
butyl)phenyl)porphyrinyl)carboxypropionyloxy)-ethoxy)-ethoxy)-
ethoxy)-ethyl)benzo[lmn]-[3,8]phenanthroline-1,3,6,8-tetraone 16
To dicarboxylic acid thread unit (50 mg, 6.11 × 10−5 mol) and
2,4-dinitrophenol (22 mg, 12.20 × 10−5 mol) in 8 mL of dry DCM
was added dicyclohexylcarbodiimide (DCC) (0.032 g, 15.28 ×
10−5 mol) and the mixture was stirred overnight at RT (under N2,
dark). The crude mixture was then washed with ice cold water,
partitioned with DCM and dried (Na2SO4). Excess solvent was
removed in vacuo. The residue was dissolved in DCM and run
through a short plug of silica using 10% MeOH–DCM as the
eluant to isolate the by-product urea derivative. A final purification
was carried out on a preparative TLC silica plate using 5% MeOH–
DCM as the eluant to obtain 16 as a yellow oil (23.9 mg, 34%);
1H NMR (300 MHz, CDCl3) d 8.92 (2H, s, Hx), 8.72 (4H, s, ND),
8.51 (2H, d, J 3, Hy), 7.54 (2H, d, J 9, Hz), 4.42 (4H, t, J 5, H26),
4.23 (4H, t, J 5, H33), 3.82 (4H, t, J 5, H27), 3.67–3.56 (20H,
m, H28, 29, 30, 31, 32), 2.98 (4H, t, J 2, H35), 2.79 (4H, t, J 2,
H34).
18 as a◦purple solid (from MeOH–CHCl3) (29.0 mg, 75%); mp
136–141 C; m/z (ES-MS) (C100H110N6O14): [M + Na]+ 1641.801
(calcd. 1641.798), [M + H]+ 1619.811 (calcd. 1619.816); 1H NMR
(300 MHz, CDCl3) d 8.84–8.82 (8H, m, H2, 3, 7, 8, 12, 13, 17,
18), 8.73–8.70 (3H, m, ortho1), 7.80–7.78 (3H, m, ortho2), 7.76–
7.73 (3H, m, para), 7.71 (1H, s, H21), 7.64–7.58 (2H, m, H22, 23),
7.49 (1H, d, J 9, H24), 6.94 (2H, d, J 8, ND2), 6.86 (2H, d, J 8,
ND2), 4.19 (2H, t, J 9, H26), 4.09–3.99 (8H, m, H29, 38, 41, 46),
3.60–3.56 (4H, m, H27, 40), 3.46 (2H, d, J 9, H39), 3.38 (2H, d, J
9, H28), 3.10 (2H, d, J 9, H35), 2.85 (2H, d, J 9, H34), 2.47 (4H, s,
H36, 37), 1.74 (27H, s, t-butyl1), 1.44 (27H, s, t-butyl2), 1.20 (3H,
t, J 6, H47), −4.13 (2H, s, H25).
Conclusions
2,7-Bis(2-(2-(3-carboxypropionyloxy)-ethoxy)-ethyl)benzo[lmn]-
Charge-transfer interactions resulting from p–p stacking of naph-
thodiimides with meso-substituted porphyrins are significant in
both intra- and inter-molecular systems. In the absence of more
strongly competitive influences, such as aromatic solvents or steric
interference through either coordinated ligands in metallopor-
phyrins, or facially encumbered porphyrins, these interactions
are maintained in solution, and are enhanced at lower tem-
peratures. In flexibly linked porphyrin–naphthodiimide (POR–
NDI–POR) systems, these interactions lead to distinct S-shaped
foldamers, whereas more rigid systems exhibit intermolecular
stacking, which is concentration and temperature dependent.
[3,8]phenanthroline-1,3,6,8-tetraone 22
A
mixture of 2,7-bis(2-(2-hydroxyethoxy)-ethyl)benzo[lmn]-
[3,8]phenanthroline-1,3,6,8-tetraone (104 mg, 0.24 mmol), 4-
(N,N-dimethylamino)pyridine DMAP (2.9 mg, 23.5 lmol,
10% mol), dry CHCl3 (16 mL), Et3N (70.8 mg), succinic anhydride
(0.278 g, 2.8 mmol) and THF (3 mL) was stirred for 3 d. The
solvent was removed in vacuo, the residue taken up in DCM, and
washed with HCl (dil. aq), water, and dried (Na2SO4). Removal
of excess solvent gave 21 as a pinkish yellow solid (150 mg,
quantitative); mp 156–159 ◦C; m/z (ES-MS) (C30H30N2O14): [M +
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These interactions can be used to advantage in the design and
construction of supramolecular systems containing these entities,
either as a template motif in their assembly, or as a recognition
element in the final system. They have considerable potential
in supramolecular assemblies involving porphyrins, especially
dynamic and controllable systems that utilise the rich photo- and
electro-chemistry available in the diversity of metalloporphyrins.
Neutral and charged systems incorporating rotaxanes, catenanes,
nanotubes and helicates can be designed using this motif, in both
thermodynamically- and kinetically-controlled self-assembly. For
viable working devices, their attachment to surfaces and interfaces
has already been demonstrated, and will no doubt be the subject
of ongoing research.
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Acknowledgements
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